This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Co-chaperonin proteins 10 (cpn10;GroES in Escherichia coli) are ring-shaped heptameric proteins that facili-tate substrate folding when in complex with cpn60 (GroEL in E. coli). Cpn10 from the hyper-thermophilic, an-cient bacterium, Aquifex aeolicus (Aacpn10) has a C-terminal 25-residue extension in each monomer not found in any other cpn10 protein. Earlier in vitro work has shown that this tail is not needed for heptamer as-sembly or protein function. Without the tail, however, the heptamers (Aacpn10del-25) readily aggregate into fibrillar stacked rings (Luke et al., 2005). To explain this phenomenon, Wittung-Stafshede group performed binding experiments with a peptide construct of the tail to establish its specificity for Aacpn10del-25 and proposed to use cryo-EM to determine its structure.